Probably not - plugins are always evolving and are now decoupled from the portal releases, and distributed on Marketplace, and have their own versions. So having a single tag for all plugins will make less sense as time goes on. I wonder if git has a way to tag subtrees, as now each plugin is decoupled from portal and from other plugins. Probably not.

I'm kind of wondering how unbundled portlet source code will be managed. I see them in the Marketplace but I don't think all of them are in github. Where would the source code be?

There shouldn't be any Liferay-authored apps on Marketplace that do not have source code on github (except for the EE-only apps). If there is, that's a bug and will be fixed, but I don't think so.. if you find any, let me know!

I think what's confusing about your statement is that there is a 6.2.0m1 tag for plugins. This seems contradictory to your statement.https://github.com/liferay/liferay-plugins/tags

To be honest, we're still trying to find the best way to manage the versioning and release policy for individual, independent plugins. Right now they're all in liferay-plugins repo, but they can now evolve independently. So I talked to bchan and he has added the 6.1.1 tag for liferay-plugins. As long as they are all in a single repo and are all updated in unison for each portal release, it makes sense (and is useful) to have a portal version tag for plugins, but the plugins themselves have their own versions now (and most will not be published along with a Liferay release - instead, they'll be uploaded on Marketplace). So it's more like a convenient timestamp for when a portal was released, to indicate that the plugins were compatible with portal at that tag level.